Yes, it was very strange that the ‘error CSS’ appeared on the site (and there were really many CSS rules here) – but in the Developer Tools it also said that it was error code. I have absolutely no idea where it came from – and I’m sure that there were CSS rules in it that I never have used on my site (eg. some font-families I didn’t even know of). Apparently it was because of this error CSS that a 500 Internal Server Error was produced causing most of my plugins to stop working properly. At the very moment I deleted the old code in the child theme footer.php – all this error CSS was gone – and my website worked completely normally again.